Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K3/00—Use of inorganic substances as compounding ingredients
  • C08K3/34—Silicon-containing compounds
  • C08K3/36—Silica
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L7/00—Compositions of natural rubber
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B60—VEHICLES IN GENERAL
  • B60C—VEHICLE TYRES; TYRE INFLATION; TYRE CHANGING; CONNECTING VALVES TO INFLATABLE ELASTIC BODIES IN GENERAL; DEVICES OR ARRANGEMENTS RELATED TO TYRES
  • B60C1/00—Tyres characterised by the chemical composition or the physical arrangement or mixture of the composition
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K13/00—Use of mixtures of ingredients not covered by one single of the preceding main groups, each of these compounds being essential
  • C08K13/02—Organic and inorganic ingredients
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K3/00—Use of inorganic substances as compounding ingredients
  • C08K3/02—Elements
  • C08K3/04—Carbon
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K5/00—Use of organic ingredients
  • C08K5/16—Nitrogen-containing compounds
  • C08K5/17—Amines; Quaternary ammonium compounds
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K5/00—Use of organic ingredients
  • C08K5/16—Nitrogen-containing compounds
  • C08K5/17—Amines; Quaternary ammonium compounds
  • C08K5/18—Amines; Quaternary ammonium compounds with aromatically bound amino groups
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K5/00—Use of organic ingredients
  • C08K5/16—Nitrogen-containing compounds
  • C08K5/34—Heterocyclic compounds having nitrogen in the ring
  • C08K5/3467—Heterocyclic compounds having nitrogen in the ring having more than two nitrogen atoms in the ring
  • C08K5/3477—Six-membered rings
  • C08K5/3492—Triazines
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B60—VEHICLES IN GENERAL
  • B60C—VEHICLE TYRES; TYRE INFLATION; TYRE CHANGING; CONNECTING VALVES TO INFLATABLE ELASTIC BODIES IN GENERAL; DEVICES OR ARRANGEMENTS RELATED TO TYRES
  • B60C1/00—Tyres characterised by the chemical composition or the physical arrangement or mixture of the composition
  • B60C1/0016—Compositions of the tread
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L2312/00—Crosslinking
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L9/00—Compositions of homopolymers or copolymers of conjugated diene hydrocarbons

Definitions

• This invention relates generally to rubber compositions and more particularly, to rubber compositions having low resin cross-linking, methods for making them and articles made from such rubber compositions.
• Tires and other articles that are made of rubber are manufactured from rubber compositions that include rubber, e.g., natural rubber, synthetic rubber or combinations thereof, as well as fillers, plasticizers, vulcanizing agents and other chemicals that improve the physical characteristics of the cured rubber composition.
• One class of materials that may be added to the rubber compositions is resin.
• Resins are typically (but not always) nonvolatile, solid organic substances that are produced naturally by plants or synthetically from petrochemicals or other sources of hydrocarbon materials. As used in rubber compositions, resins may be classified as either reinforcing resins or as plasticizing resins. Plasticizing resins are added to a rubber composition to improve the plasticity or workability of a rubber composition. They are often added as a substitute for or in addition to a processing oil and are known to improve the resulting physical characteristics of the cured rubber composition.
• Reinforcing resins are added to a rubber composition to increase the rigidity of the cured rubber composition. These reinforcing resins intermix with the rubber polymer chains and, when reacted with a linking agent or with each other, form a three-dimensional network that improves the physical characteristics of the cured rubber composition. Many of these resins are classified as methylene acceptor/donor systems that react together to generate a three-dimensional reinforcing resin network by a condensation reaction.
• additives are known to be added to rubber compositions to improve their physical properties, such additives including antioxidants, antiozonants, anti-reversion agents and so forth. Such additives are well known in the rubber industry.
• Particular embodiments of the present invention include rubber compositions, articles made from such rubber compositions and methods for making same.
• Such embodiments include a tire component, the tire component comprising a rubber composition that is based upon a cross-linkable elastomer composition, the cross-linkable elastomer composition comprising, per 100 parts by weight of rubber (phr) a highly unsaturated diene elastomer, a reinforcing filler and a methylene acceptor selected from 3-hydroxydiphenylamine, 4-hydroxydiphenyl-amine or combinations thereof.
• Such rubber compositions further include a methylene donor, wherein a ratio of the methylene acceptor to the methylene donor may be at least 15:1.
• the ratio of the methylene acceptor to the methylene donor may in particular embodiments may be between 15:1 and 50:1 or between 15:1 and 30:1.
• Methods that are embodiments of the present invention include methods for manufacturing a tire component, such methods comprising mixing together components of a rubber composition into a non-productive mix, the components including a highly unsaturated diene elastomer, a reinforcing filler and a methylene acceptor selected from 3-hydroxydiphenylamine, 4-hydroxydiphenyl-amine or combinations thereof.
• Such methods may further include cooling the non-productive mix and mixing a methylene donor and a vulcanizing agent into the non-productive mix to convert the non-productive mix to a productive mix.
• the method may further include forming the tire component from the productive mix.
• Particular embodiments of the present invention include rubber compositions that contain specific methylene acceptors useful in methylene acceptor/donor systems that are typically used as reinforcing resins in rubber compositions.
• the specific methylene acceptors are selected from 3-hydroxydiphenylamine (3-HDPA), 4-hydroxydiphenylamine (4-HDPA) or combinations thereof.
• 3-hydroxydiphenylamine 3-hydroxydiphenylamine
• 4-HDPA 4-hydroxydiphenylamine
• iene elastomer and “rubber” are synonymous terms and may be used interchangeably.
• a “non-productive” mix includes many of the components of a rubber composition but includes no vulcanization agents and typically no primary accelerators.
• a “productive” mix results after the vulcanization agents and typically any primary accelerators are added to the non-productive mix.
• HDPA is a generic term referring to 3-HDPA, 4-HDPA and combinations thereof.
• based upon is a term recognizing that embodiments of the present invention are made of vulcanized or cured rubber compositions that were, at the time of their assembly, uncured.
• the cured rubber composition is therefore “based upon” the uncured rubber composition.
• the cross-linked rubber composition is based upon or comprises the constituents of the cross-linkable rubber composition.
• the rubber compositions disclosed herein are useful for many types of articles that are made of rubber including tires, rubber coatings, vibration dampeners, tool grips, tools and so forth.
• Tires are made of many rubber parts and the rubber compositions disclosed herein are useful for such tire components as, for example, sidewall components, treads, undertreads and/or rubber coatings for the cords, plies and/or belts.
• the rubber compositions disclosed herein that have the surprising improvement in the rubber's ability to deter damage from ozone and fatigue failure include an HDPA methylene acceptor with a very small quantity of methylene donor.
• Methylene acceptors and methylene donors and their use in methylene acceptor/donor systems are well known in the art for their ability to increase the rigidity of a rubber composition.
• the methylene donor reacts with the methylene acceptor to generate a three-dimensional reinforcing resin network by a condensation reaction that, when formed in the rubber composition, provides it, inter alia, with increased rigidity.
• Suitable methylene donors that will react with the HDPA may be selected, for example, from hexamethylenetetramine (HMT); hexamethoxymethylmelamine (HMMM); formaldehyde; paraformaldehyde; trioxane; 2-methyl-2-nitro-1-propanal; substituted melamine resins such as N-substituted oxymethylmelamine resins; glycoluril compounds such as tetramethoxymethyl glycoluril; urea-formaldehyde resins such as butylated urea-formaldehyde resins; or mixtures thereof.
• Hexamethylenetetramine (HMT), hexamethoxymethylmelamine (HMMM) or mixtures thereof are preferred methylene donors in particular embodiments.
• the amount of the HDPA added to the rubber compositions for particular embodiments disclosed herein may be between 1 phr and 20 phr or alternatively between 2 phr and 15 phr, 1 phr and 10 phr, or between 3 and 10 phr.
• the methylene donor may be added in amount such that the ratio of the methylene acceptor to the methylene donor at least 15:1 or alternatively, between 15:1 and 50:1, between 15:1 and 40:1, between 15:1 and 30:1, between 20:1 and 25:1 or between 20:1 and 40:1.
• the HDPA may be added to the rubber composition as 3-hydroxydiphenylamine (3-HDPA), 4-hydroxydiphenylamine (4-HDPA) or as combinations thereof.
• 3-HDPA is available from Santa Cruz Biotechnology, Inc. with offices in California and 4-HDPA is available from NeoSources International with offices in Texas.
• the rubber compositions disclosed herein may include just the HDPA with no methylene donor.
• the total amount of the methylene acceptor will be less than 0.5 phr or alternatively, between 0.05 phr and 0.45 phr, between 0.1 phr and 0.45 phr, between 0.2 phr and 0.4 phr or between 0.2 phr and 0.3 phr.
• the useful elastomers of the rubber composition disclosed herein include highly unsaturated diene elastomers.
• Diene elastomers or rubber is understood to mean those elastomers resulting at least in part (i.e., a homopolymer or a copolymer) from diene monomers (monomers bearing two double carbon-carbon bonds, whether conjugated or not).
• Essentially unsaturated diene elastomers are understood to mean those diene elastomers that result at least in part from conjugated diene monomers, having a content of members or units of diene origin (conjugated dienes) that are greater than 15 mol. %.
• diene elastomers such as butyl rubbers, nitrile rubbers or copolymers of dienes and of alpha-olefins of the ethylene-propylene diene terpolymer (EPDM) type or the ethylene-vinyl acetate copolymer type, do not fall within the preceding definition and may in particular be described as “essentially saturated” diene elastomers (low or very low content of units of diene origin, i.e., less than 15 mol. %).
• Particular embodiments of the present invention include no essentially saturated diene elastomers.
• essentially unsaturated diene elastomers are the highly unsaturated diene elastomers, which are understood to mean in particular diene elastomers having a content of units of diene origin (conjugated dienes) that is greater than 50 mol. %.
• Particular embodiments of the present invention may include not only no essentially saturated diene elastomers but also no essentially unsaturated diene elastomers that are not highly unsaturated.
• the rubber elastomers suitable for use with particular embodiments of the present invention include highly unsaturated diene elastomers, for example, polybutadienes (BR), polyisoprenes (IR), natural rubber (NR), butadiene copolymers, isoprene copolymers and mixtures of these elastomers.
• the polyisoprenes include synthetic cis-1,4 polyisoprene, which may be characterized as possessing cis-1,4 bonds at more than 90 mol. % or alternatively, at more than 98 mol. %.
• rubber elastomers that are copolymers and include, for example, butadiene-styrene copolymers (SBR), butadiene-isoprene copolymers (BIR), isoprene-styrene copolymers (SIR) and isoprene-butadiene-styrene copolymers (SBIR) and mixtures thereof.
• SBR butadiene-styrene copolymers
• BIR butadiene-isoprene copolymers
• SIR isoprene-styrene copolymers
• SBIR isoprene-butadiene-styrene copolymers
• any of the highly unsaturated elastomers may be utilized in particular embodiments as a functionalized elastomer.
• Elastomers can be functionalized by reacting them with suitable functionalizing agents prior to or in lieu of terminating the elastomer.
• suitable functionalizing agents include, but are not limited to, metal halides, metalloid halides, alkoxysilanes, imine-containing compounds, esters, ester-carboxylate metal complexes, alkyl ester carboxylate metal complexes, aldehydes or ketones, amides, isocyanates, isothiocyanates, imines, and epoxides.
• functionalized elastomers are known to those of ordinary skill in the art. While particular embodiments may include one or more of these functionalized elastomers solely as the rubber component, other embodiments may include one or more of these functionalized elastomers mixed with one or more of the non-functionalized highly unsaturated elastomers.
• a reinforcing filler is included in the rubber compositions disclosed herein.
• Reinforcing fillers are well known in the art and include, for example, carbon blacks and silica. Any reinforcing filler known to those skilled in the art may be used in the rubber composition either by themselves or in combination with other reinforcing fillers.
• the filler is essentially a carbon black.
• Carbon black which is an organic filler, is well known to those having ordinary skill in the rubber compounding field.
• the carbon black included in the rubber compositions produced by the methods disclosed herein may, in particular embodiments for example, be in an amount of between 40 phr and 150 phr or alternatively between 50 phr and 100 phr.
• Suitable carbon blacks are any carbon blacks known in the art and suitable for the given purpose. Suitable carbon blacks of the type HAF, ISAF and SAF, for example, are conventionally used in tire treads. Non-limitative examples of carbon blacks include, for example, the N115, N134, N234, N299, N326, N330, N339, N343, N347, N375 and the 600 series of carbon blacks, including, but not limited to N630, N650 and N660 carbon blacks.
• silica may also be useful as reinforcement filler.
• the silica may be any reinforcing silica known to one having ordinary skill in the art including, for example, any precipitated or pyrogenic silica having a BET surface area and a specific CTAB surface area both of which are less than 450 m 2 /g or alternatively, between 30 and 400 m 2 /g may be suitable for particular embodiments based on the desired properties of the cured rubber composition.
• Particular embodiments of rubber compositions disclosed herein may include a silica having a CTAB of between 80 and 200 m 2 /g, between 100 and 190 m 2 /g, between 120 and 190 m 2 /g or between 140 and 180 m 2 /g.
• the CTAB specific surface area is the external surface area determined in accordance with Standard AFNOR-NFT-45007 of November 1987.
• Highly dispersible precipitated silicas may be useful in particular embodiments of such rubber compositions disclosed herein, wherein “highly dispersible silica” is understood to mean any silica having a substantial ability to disagglomerate and to disperse in an elastomeric matrix. Such determinations may be observed in known manner by electron or optical microscopy on thin sections.
• Examples of known highly dispersible silicas include, for example, Perkasil KS 430 from Akzo, the silica BV3380 from Degussa, the silicas Zeosil 1165 MP and 1115 MP from Rhodia, the silica Hi-Sil 2000 from PPG and the silicas Zeopol 8741 or 8745 from Huber.
• silane coupling agent is a sulfur-containing organosilicon compound that reacts with the silanol groups of the silica during mixing and with the elastomers during vulcanization to provide improved properties of the cured rubber composition.
• a suitable coupling agent is one that is capable of establishing a sufficient chemical and/or physical bond between the inorganic filler and the diene elastomer; which is at least bifunctional, having, for example, the simplified general formula “Y-T-X”, in which: Y represents a functional group (“Y” function) which is capable of bonding physically and/or chemically with the inorganic filler, such a bond being able to be established, for example, between a silicon atom of the coupling agent and the surface hydroxyl (OH) groups of the inorganic filler (for example, surface silanols in the case of silica); X represents a functional group (“X” function) which is capable of bonding physically and/or chemically with the diene elastomer, for example by means of a sulfur atom; T represents a divalent organic group making it possible to link Y and X.
• Y represents a functional group (“Y” function) which is capable of bonding physically and/or chemically with the inorganic
• any of the organosilicon compounds that contain sulfur and are known to one having ordinary skill in the art are useful for practicing embodiments of the present invention.
• suitable silane coupling agents having two atoms of silicon in the silane molecule include 3,3′-bis(triethoxysilylpropyl) disulfide and 3,3′-bis(triethoxy-silylpropyl) tetrasulfide (known as Si69). Both of these are available commercially from Degussa as X75-S and X50-S respectively, though not in pure form. Degussa reports the molecular weight of the X50-S to be 532 g/mole and the X75-S to be 486 g/mole.
• Both of these commercially available products include the active component mixed 50-50 by weight with a N330 carbon black.
• suitable silane coupling agents having two atoms of silicon in the silane molecule include 2,2′-bis(triethoxysilylethyl) tetrasulfide, 3,3′-bis(tri-t-butoxy-silylpropyl) disulfide and 3,3′-bis(di t-butylmethoxysilylpropyl) tetrasulfide.
• silane coupling agents having just one silicon atom in the silane molecule include, for example, 3,3′(triethoxysilylpropyl) disulfide and 3,3′ (triethoxy-silylpropyl) tetrasulfide.
• the amount of silane coupling agent can vary over a suitable range as known to one having ordinary skill in the art. Typically the amount added is between 7 wt. % and 15 wt. % or alternatively between 8 wt. % and 12 wt. % or between 9 wt. % and 11 wt. % of the total weight of silica added to the rubber composition.
• Particular embodiments of the rubber compositions disclosed herein may include no processing oil or very little, such no more than 5 phr.
• Processing oils are well known to one having ordinary skill in the art, are generally extracted from petroleum and are classified as being paraffinic, aromatic or naphthenic type processing oil, including MES and TDAE oils. Processing oils are also known to include, inter alia, plant-based oils, such as sunflower oil, rapeseed oil and vegetable oil.
• Some of the rubber compositions disclosed herein may include an elastomer, such as a styrene-butadiene rubber, that has been extended with one or more such processing oils but such oil is limited in the rubber composition of particular embodiments as being no more than 10 phr of the total elastomer content of the rubber composition.
• the rubber compositions disclosed herein may further include, in addition to the compounds already described, all or part of the components often used in diene rubber compositions intended for the manufacture of tires, such as plasticizers, pigments, protective agents of the type that include antioxidants and/or antiozonants, vulcanization retarders, a vulcanization system based, for example, on sulfur or on a peroxide, vulcanization accelerators, vulcanization activators, extender oils and so forth. There may also be added, if desired, one or more conventional non-reinforcing fillers such as clays, bentonite, talc, chalk or kaolin.
• the vulcanization system is preferably, for particular embodiments, one based on sulfur and on an accelerator but other vulcanization agents known to one skilled in the art may be useful as well.
• Vulcanization agents as used herein are those materials that cause the cross-linkage of the rubber and therefore may be added only to the productive mix so that premature curing does not occur, such agents including, for example, sulfur and peroxides.
• Use may be made of any compound capable of acting as an accelerator of the vulcanization of elastomers in the presence of sulfur, in particular those chosen from the group consisting of 2-mercaptobenzothiazyl disulfide (abbreviated to “MBTS”), N-cyclohexyl-2-benzothiazolesulphenamide (abbreviated to “CBS”), N,N-dicyclohexyl-2-benzothiazolesulphenamide (abbreviated to “DCBS”), N-tert-butyl-2-benzothiazolesulphenamide (abbreviated to “TBBS”), N-tert-butyl-2-benzothiazolesulphenimide (abbreviated to “TBSI”) and the mixtures of these compounds.
• MBTS 2-mercaptobenzothiazyl disulfide
• CBS N-cyclohexyl-2-benzothiazolesulphenamide
• DCBS N,N-dicyclohexyl-2-benzo
• the vulcanization system may further include various known secondary accelerators or vulcanization activators, such as zinc oxide, stearic acid and guanidine derivatives (in particular diphenylguanidine).
• various known secondary accelerators or vulcanization activators such as zinc oxide, stearic acid and guanidine derivatives (in particular diphenylguanidine).
• the rubber compositions that are embodiments of the present invention may be produced in suitable mixers in a manner known to those having ordinary skill in the art. Typically the mixing may occur using two successive preparation phases, a first phase of thermo-mechanical working at high temperature followed by a second phase of mechanical working at a lower temperature.
• the first phase sometimes referred to as a “non-productive” phase, includes thoroughly mixing, for example by kneading in a Banbury type mixer, the various ingredients of the composition but excluding the vulcanization system and the methylene donor. It is carried out in a suitable kneading device, such as an internal mixer, until under the action of the mechanical working and the high shearing imposed on the mixture, a maximum temperature of generally between 120° C. and 190° C. is reached.
• a suitable kneading device such as an internal mixer
• this finishing phase consists of incorporating the vulcanization system and the methylene donor into the rubber composition using a suitable device, such as an open mill. It is performed for an appropriate time (typically, for example, between 1 and 30 minutes or between 2 and 10 minutes), and at a sufficiently low temperature, i.e., lower than the vulcanization temperature of the mixture and lower than the cross-linking temperature of the methylene donor/acceptor system, so as to protect against premature vulcanization or resinification cross-linking.
• the rubber composition can be formed into useful articles, including tire components.
• Tire treads for example, may be formed as tread bands and then later made a part of a tire or they be formed directly onto a tire carcass by, for example, extrusion and then cured in a mold.
• Other components such as those located in the bead area of the tire or in the sidewall may be formed and assembled into a green tire and then cured with the curing of the tire.
• fabric or metal may be coated with the rubber compositions for later incorporation into a tire building process as, for example, tire belts or cords.
• Moduli of elongation were measured at 10% (MA10) and at 100% (MA100) at a temperature of 23° C. based on ASTM Standard D412 on dumb bell test pieces. The measurements were taken in the second elongation; i.e., after an accommodation cycle. These measurements are secant moduli in MPa, based on the original cross section of the test piece.
• the elongation property was measured as elongation at break (%) and the corresponding elongation stress (MPa), which is measured at 23° C. in accordance with ASTM Standard D412 on ASTM C test pieces.
• Tear properties were determined from test samples cut from a cured plaque with a thickness of approximately 2.5 mm. Notches (perpendicular to the test direction) were created in the samples prior to testing. The force and elongation at break was measured using an Instron 5565 Uniaxial Testing System. The cross-head speed was 500 mm/min. Samples were tested at ambient and at 60° C. and at 100° C.
• Static ozone surface cracking was evaluated using a test closely related to the ASTM 1149-99 Standard Test Method for rubber deterioration titled Surface Ozone Cracking in a Chamber.
• the testing utilized in the examples that follow differs in the construction of the sample holder, which was a rod rather than a wooden block holder as required under the ASTM test method. Rectangular samples were cut with a die then folded in half and stapled such that the curvature of the loop has a maximum local strain of 18%.
• the samples were hung on a rod for 2-5 days under ambient conditions before being placed in an ozone chamber.
• the ozone chamber conditions were set at 50 parts per hundred million ozone (pphm) and a temperature of 40° C. for one week.
• the samples were periodically evaluated for cracks.
• the samples were evaluated using the Rubber Deterioration Test Grades that consists of three numbers. The first number indicates the number of cracks in the sample, the second rates the width of the cracks and the third number is the depth of the crack. Zero indicates that no cracks were observed.
• the ozone cracking index is the product of the three numbers determined by the Rubber Deterioration Test Grades.
• Cyclic fatigue testing was conducted on the cured test samples that were dumbbell shaped and 65 mm long. The test was conducted at ambient temperature by imposing cyclic strain from between 0% and 60% on the test pieces. The total number of cycles to failure was recorded for each test sample as the measurement of cyclic fatigue.
• This example illustrates the surprising increase in the resistance to ozone attack and fatigue failure by the rubber compositions in accordance with the present invention.
• the formulations were prepared with the component amounts shown in Table 1.
• the 4-HDPA was obtained from NeoSources International with offices in Texas. It should be noted that no anti-degradation chemicals were added to the rubber compositions that would mask the results achieved from the 4-HDPA.
• the natural rubber and all the other materials except for the sulfur, accelerators and the hexamethylenetetramine were added to a Bandury mixer and processed until well incorporated. The mixture was then dropped from the mixer, transferred to a mill and cooled.

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